In UMTS (Universal Mobile Telecommunications System) networks, for the purpose of improving spectral efficiency and further improving data rates, by adopting HSDPA (High Speed Downlink Packet Access) and HSUPA (High Speed Uplink Packet Access), it is performed exploiting maximum features of the system based on W-CDMA (Wideband Code Division Multiple Access). For the UMTS network, for the purpose of further increasing high-speed data rates, providing low delay and the like, Long Term Evolution (LTE) has been studied.
In the 3G system, a fixed band of 5 MHz is substantially used, and it is possible to achieve transmission rates of approximately maximum 2 Mbps in downlink. Meanwhile, in the LTE-scheme system, using variable bands ranging from 1.4 MHz to 20 MHz, it is possible to achieve transmission rates of maximum 300 Mbps in downlink and about 75 Mbps in uplink. Further, in the UMTS network, for the purpose of further increasing the wide-band and high speed, successor systems to LTE have been studied (for example, LTE Advanced (LTE-A)). For example, in LTE-A, it is scheduled to increase 20 MHz that is the maximum system band in LTE specifications to about 100 MHz. Further, it is scheduled to increase four antennas that are the maximum number of transmission antennas in LTE specification to eight antennas.
Moreover, in the LTE-scheme system (LTE system), MIMO (Multi Input Multi Output) systems are proposed as radio communication techniques for transmitting and receiving data using a plurality of antennas and improving a data rate (spectral efficiency) (for example, see Non-patent Document 1). In the LTE system, two modes, spatial multiplexing transmission mode (SU-MIMO (Single User MIMO)) and transmission diversity transmission mode, are defined as downlink MIMO transmission modes. The spatial multiplexing transmission mode is to spatially multiplex signals of a plurality of streams in the same frequency and time to transmit, and is effective at increasing a peak data rate. The transmission diversity transmission mode is to transmit signals of the same stream subjected to space-frequency (time) coding from a plurality of antennas, and is effective at improving reception quality of a cell-edge user due to the transmission antenna diversity effect.
In such a MIMO system, proposed are techniques (rank adaptation) for optimally controlling the number of layers of spatial multiplexing corresponding to reception conditions in each mobile station receiver. In the rank adaptation, based on downlink channel information (reception SIRN, inter-antenna fading correlation), a base station transmitter performs control for transmitting information to a mobile station receiver good in the channel state by the spatial multiplexing transmission mode, while transmitting information to a mobile station receiver poor in the channel state by the transmission diversity transmission mode.